The present invention relates to a blocked organic polyisocyanate, a method of producing it and one-component polyurethane systems containing it.
Blocking of polyisocyanates to effect temporary protection of the isocyanate groups thereof is a procedure which has long been known, and is described, for example, in Houben Weyl, Methoden der organischen Chemie XIV/2, pages 61-70. Hardenable compositions which contain blocked polyisocyanates are used in polyurethane coating compositions, for example.
One-component (1K) polyurethane systems are widely used in the field of industrial stoving lacquers such as mass-production automobile coatings and coil coatings, and have very good film properties, such as resistance to chemicals, scratch-resistance and resistance to weathering. These lacquer films are hardened by thermal activation (by a stoving operation) of the blocked polyisocyanates with polyols, optionally in the presence of a suitable catalyst. A review of suitable blocking agents is given by Wicks et al. in Progress in Organic Coatings 1975, 3, pages 73-79, 1981, 9, pages 3-28 and 1999, 36, pages 148-172, for example.
In the field of automobile coating, the blocked polyisocyanates must be crosslinkable at maximum stoving temperatures of 140xc2x0 C., and must only exhibit very slight yellowing, and preferably no yellowing, during the stoving operation. The stoving temperature is mainly controlled via the reactivity of the blocked polyisocyanate.
Most stoving systems, such as melamine-formaldehyde and urea-formaldehyde resins, for example, can release of volatile constituents during hardening, which increase the VOC value.
Moreover, a certain proportion of the blocking agent remains in the lacquer film which is formed and has a disadvantageous effect on the properties thereof. Due to the remaining blocking agent, properties such as the scratch-resistance and acid-resistance of one-component lacquer films are not comparable with two-component (2K) polyurethane lacquer coatings (e.g. T. Engbert, E. Kxc3x6nig, E. Jxc3xcrgens, FarbeandLack, Curt R. Vincentz Verlag, Hannover 10/1995). Furthermore, separation of the blocking agent and the escape thereof in gaseous form from the lacquer film can lead to bubble formation in the lacquer film. Subsequent incineration of the emitted blocking agent can sometimes be necessary.
Isocyanates, blocked with diethyl malonate, have mainly been used recently for particularly low stoving temperatures within the range from 90 to 120xc2x0 C. (e.g. EP-A 0947531). In contrast to blocking procedures which employ heterocyclic N compounds, such as caprolactam or butanone oxime, for example, the blocking agent as a whole is not split off or separated here; rather, this blocking agent results in a transesterification reaction on the isocyanate which is blocked with diethyl malonate. Ethanol is separated during this transesterification. This method can be employed at relatively low stoving temperatures, since the second, adjacent ester function is an activated ester. The disadvantage of this method is that systems such as these are extremely susceptible to the effect of acids, because the labile ester bond can be rapidly cleaved. The possibilities for the use of these products are thereby restricted.
It is an object of the present invention to provide new blocked polyisocyanate systems which react without separation of the blocking agent, i.e. free from emissions, and which exhibit low crosslinking temperatures. It is another object that these blocked polyisocyanate systems should be stable on storage at ambient temperature, and that they should be suitable, particularly in combination with suitable polyol components, for the production of one-component stoving lacquers.
Surprisingly, it has now been found that acidic CH compounds which possess the basic structure of an activated cyclic ketone, particularly that of cyclopentanone-2-carboxymethyl ester, are particularly suitable for blocking polyisocyanates in order to obtain emission-free coatings with a reduced tendency to exhibit yellowing.
The present invention relates to a blocked organic polyisocyanate having at least two isocyanate groups, which are blocked with acidic CH cyclic ketones of formula (I), 
wherein
X is an electron-attracting group,
R1 and R2, independently of each other, represent the radicals H, a C1-C20 (cyclo)alkyl, a C6-C24 aryl, a C1-C20 (cyclo)alkyl ester or amide, a C6-C24 aryl ester or amide, or mixed aliphatic/aromatic radicals having 1 to 24 carbon atoms, which can also form part of a 4 to 8-membered ring,
n is an integer from 0 to 5,
and which have a content of blocked isocyanate groups (calculated as NCO) of 20 to 0% by weight in total.